DIAGNOSIS The clutch is slipping or the drive sprocket is worn out. This is a mechanical failure due to normal wear or a loose chain. A dull chain exacerbates this by increasing load.
FIX First, check chain tension. A properly tensioned chain should snap back against the bar when pulled from the underside. If tension is correct, inspect the clutch and sprocket. Remove the side cover and clutch cover nut. The clutch drum and rim sprocket should be replaced as a set if the sprocket teeth are visibly hooked or worn down. Part # 503847001 for the rim sprocket kit.
IF THAT DIDN'T WORK The clutch springs have lost tension or the clutch shoes are glazed. The entire clutch assembly (part # 501879201) must be replaced. You need a clutch removal tool to compress and remove the clutch from the crankshaft. Ensure the chain brake band is not partially engaged and dragging.
HEADS UP A dull chain is the leading cause of clutch and sprocket wear. Always cut with a sharp chain. Never run the saw with the chain brake engaged, as this destroys the clutch.